Many wireless communications systems contain a base station in two-way wireless communication with a plurality of wireless transmit/receive units (WTRUs). The base station may send signals containing beamforming vectors to each WTRU. The signal instructs the WTRU as to how to receive a radio beam formed by a base station having a specific beam shape for communicating between the WTRU and the base station. A goal of such beam forming is to optimize the overall performance of the system. One example of such optimization is the supporting of multi-user multiple-input multiple-output (MU-MIMO) communications and minimizing of interference when two or more WTRUs are transmitting simultaneously using the same frequency/time resources.
Co-pending U.S. patent application Ser. No. 10/052,842, which is incorporated herein by reference, discloses that beamforming or precoding information needs to be communicated from a transmitter, (e.g., a base station), to a receiver, (e.g., a wireless transmit/receive unit (WTRU)), to avoid a channel mismatch between transmitting and receiving signals. This is particularly important for multiple-input multiple-output (MIMO) data demodulation when beamforming and precoding are used. When a receiver uses incorrect beamforming information for constructing effective channel responses for data detection, significant performance degradation can occur.
Generally, beamforming or precoding information may be communicated using explicit control signaling, particularly when the transmitter and receiver are restricted to the use of limited sets of antenna weight coefficients for beamforming and precoding. The limited sets of antenna weight coefficients are sometimes referred to as a beamforming or precoding codebook. Explicit signaling to communicate beamforming or precoding information from a transmitter to a receiver may incur large signaling overhead, particularly for MU-MIMO systems in which the desired beamforming information needs to be communicated to the WTRU. Furthermore, interference beamforming information may have to be communicated to the WTRU to enable advanced receiver processing, such as joint detection and interference cancellation. The signaling overhead increases when a large size codebook is deployed.
FIG. 1 shows a wireless communication system 100 including a base station 105 and a WTRU 110. The base station 105 may include a MIMO antenna 115 having a plurality of transmit antennas 116A, 115B, 115C and 115D. The WTRU 110 may also include a MIMO antenna 120 having a plurality of receive antennas 120A, 120B, 120C and 120D. The base station 105 communicates with the WTRU 110 by transmitting signals via resource blocks (RBs) 125 to the WTRU 110. Each of the RBs 125 has a particular RB structure that includes a plurality of resource elements (REs). In accordance with the particular RB structure, each RE may be reserved for one of the following:
1) a common reference signal (CRS) associated with one of the transmit antennas 116A, 115B, 115C and 115D of the base station 105;
2) a dedicated reference signal (DRS) including a single beamformed or precoded pilot;
3) a DRS including a composite beamformed or precoded pilot; and
4) a data symbol.
At least a portion of data symbols reserved by REs of the RBs 125 are “control type” data symbols that include a DRS mode indicator. Once decoded, the DRS mode indicator enables the WTRU 110 to properly detect/demodulate data symbols in the RBs 125 transmitted by the base station 105.
A hybrid DRS scheme in which REs are reserved for DRSs including a single beamformed or precoded pilot and/or a composite beamformed or precoded pilot is introduced, where a plurality (N) of DRSs per RB are used.
As introduced by co-pending U.S. patent application Ser. No. 10/052,842, FIG. 2 shows an example of an RB structure that may be used to transmit signals by the base station 105 and receive signals by the WTRU. Each of a plurality of RBs 205 and 210 includes a plurality of REs reserved for data symbols (D), a plurality of REs reserved for CRSs associated with respective base station transmit antennas (T1-T4), and a plurality of REs reserved for DRSs (P), which include either a single beamformed or precoded pilot, or a composite beamformed or precoded pilot. As shown in FIG. 2, the DRSs are reserved by REs 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, 265 and 270.
Signaling of the beamforming vector of interfering WTRUs to a specific desired WTRU allows the specific WTRU to perform advanced receiver processing, (e.g. joint detection and cancellation of interference). At the same time, signaling the beamforming vector of the specific desired WTRU may require better accuracy than signaling the beamforming vectors of interfering WTRUs. Usually, the information transmitted using explicit signaling, such as by using a physical downlink control channel (PDCCH), is more accurately detected and decoded by the WTRU in terms of error probability or rate of detection than the information transmitted using implicit signaling, such as by using a DRS. This is because explicit signaling (e.g., signaling via PDCCH) is protected by channel coding and cyclic redundancy check (CRC). On the other hand, implicit signaling (e.g., signaling via DRS) does not have channel coding and CRC protection, and requires blind detection to withdraw the information carried by the DRS. However, the overhead using explicit signaling or PDCCH to carry all beamforming information including both desired and interference information is large, as compared with the overhead using implicit signaling or DRS. A more efficient signaling scheme and method is desirable to minimize the signaling overhead while maintaining the performance and at the same time have more protection on the most important beamforming information.